Optical links provide high data transmission rates at low power, and thus present a viable solution for replacing ordinary copper interconnects between integrated circuits. Optical reception is based on capturing, using a photosensitive device such as a photodiode, a light signal that is generally encoded in a digital fashion, and which may have a power level as low as 10 μW. The photosensitive device for example generates a small current that is transformed by the optical receiver into a digital voltage signal.
In order to correctly receive a data signal transmitted optically over such an optical link, it is generally necessary to receive a timing signal over the optical link. In some embodiments, the timing signal may be extracted from the data signal itself, but such solutions tend to be complex to implement. Indeed, the data encoding will generally mean that a timing edge is not present in the data signal for each data bit of the data transmitted over the link.
In order to generate a clock signal on the receive side based on a low power clock transmission, it has been proposed to use an injection-locked solution. Such a solution uses a ring oscillator oscillating at a given frequency. The received low power clock signal is injected at a node of the ring oscillator in order to modify its oscillation frequency to the desired frequency as defined by the clock signal.
A difficulty is that, due to process variations, the natural oscillating frequency of the ring oscillator may in some cases be relatively far from the desired frequency. Thus the injection of the low power clock signal may not be enough to bring the oscillation frequency of the ring oscillator to a desired frequency. A solution could be to increase the power of the transmitted clock signal, but this would lead to increased power consumption.